THE GYMNOSPERMAE : CYCADALES, ETC. 



719 



ENDARCH 



PROTO- 



XYLEM 



MESARCH 

 PROTOXYLEM 



EXARCH 

 PROTOXYLEM 



Fig. ~Z2. — Diagram to illustrate the difference 

 between the exarch, endarch and mesarch 

 positions of the protoxyiem. {After 

 Zimmermonn.) 



Fig. 723. — Cycos revohita. Transverse section of a pinna showing the palisade tissue and 

 the transversely elongated transfusion cells in the lower mesophyll. 



Anatomy of the Root 



The coralloid roots have been described above. The structure of the 

 primary tap root is ver\' Hke that of a fleshy Dicotyledon (Fig. 724), diarch 

 in its youngest parts and becoming polyarch near the base. Old roots have 

 several concentric vascular rings which develop simultaneously with the 

 same structures in the stem and they also develop a periderm. A root cap 

 is present, but it arises from the periblem and there is no true calyptrogen 

 layer. The branching of the lateral roots has the appearance of dichotomy, 

 but this arises from the arrest of their apices and the development of opposite 

 lateral roots close to the arrested apex. 



